Fig. 24.—Sketch showing
the unbalanced portion of
the Crank by Shaded Lines.
Fig. 25.—Two-throw Crankshaft balanced by
drilling Holes in Flywheel and Fan Pulley.
Fig. 26.—Sketch to illustrate the
Forces acting on a Flywheel Rim.
The Flywheel.—We have already described how the force driving the piston of a motor-car engine varies during the four strokes of the cycle, but we must note that it also varies considerably during each individual stroke. Thus, on what is known as the explosion stroke (or power stroke) of the cycle, the pressure at the commencement of this stroke may be exceedingly great and yet towards the end of the stroke the gases have expanded and the exhaust valve has been opened, so that the pressure acting on the piston is then very small. Again, on the compression, suction, and exhaust strokes, the piston has to be pushed or pulled by some means, as no power is being generated. Therefore, if the engine is to be self-acting and run continuously, some means must be provided for storing up the great force of the explosions and giving it out again on the idle strokes. The function of the flywheel is to store any energy given to it over and above that required to drive the car and to give it out again when required for performing the functions of compressing, exhausting, and sucking in gas, as well as to keep the car running steadily. It is simply a heavy wheel mounted on the end of the crankshaft which, when once started revolving at a high speed, is not easily stopped, and which will give up part of its energy each time its speed is reduced owing to the demands of the engine; but when the engine is generating power the wheel will speed up and store the excess—the mere fact that the wheel is heavy causes these changes in speed to occur slowly, and therefore on the whole the fluctuation of speed is not great when a suitable flywheel is fitted. The flywheel does not limit the maximum speed of the engine, as it could go on slowly increasing in speed if no resistance was encountered until the wheel finally burst or flew to pieces. Thus the flywheel does not regulate the speed of the engine; it merely smooths out the inequalities in the several strokes of the “cycle.” Flywheels of motor-car engines are now always made of steel, so that they can be safely run at speeds up to 3,000 revolutions per minute without fear of the rim bursting. All parts of the rim tend to fly off radially in the direction of the arrows as shown in Fig. [26] under the action of centrifugal force. A built-up flywheel is shown in Fig. [27], and one made from a single stamping of steel is shown in Fig. [28]. Generally speaking, when a coned clutch is fitted one portion of it is formed on the inside of the flywheel rim as indicated in these two figures. When the construction shown in Fig. [28] is adopted the lining would be inserted after the clutch cone had been put into place; very often the lining is made up of sections which can be readily inserted or withdrawn after the cone is in position.